_pluginfactory/pluginoriginpip.py: Fix the description string

[gtristan]

The Distribution object returned by importlib.metadata.distribution() does not have a sensible __str__() dunder method to rely on. Instead we should be using Distribution.name and Distribution.version members to describe the pip package which was loaded in the plugin description string.

This changes the following annoying output from bst build and other session commands:

    Source Plugins
        tar:      core plugin
        git_repo: junction: plugins/buildstream-plugins-community.bst (project directory: src/buildstream_plugins_community/sources)
        patch:    python package '<importlib.metadata.PathDistribution object at 0x7fc2d858cda0>' at: /usr/lib/python3.12/site-packages

To the following, more satisfying output:

    Source Plugins
        tar:      core plugin
        git_repo: junction: plugins/buildstream-plugins-community.bst (project directory: src/buildstream_plugins_community/sources)
        patch:    python package 'buildstream-plugins' version 2.5.0 at: /usr/lib/python3.12/site-packages